JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/css/bootstrap.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js"></script>
<!doctype html>
<html lang="en">
  <body class="text-center">

    <nav class="navbar navbar-expand-md navbar-light fixed-top" style="background-color: #FFFFFF;">
    
            <button class="navbar-toggler navbar-nav mr-auto" type="button" data-toggle="collapse" data-target="#navbarCollapse" aria-controls="navbarCollapse" aria-expanded="false" aria-label="Toggle navigation">
                  <span class="navbar-toggler-icon"></span> 
            </button>

            <a class="navbar-brand mx-auto" href="#">My Brand</a>

            <ul class="nav navbar-nav ml-md--auto"> 

                    <li class="dropdown"> 
                        
                        <a href="#" class="nav-link dropdown-toggle" id="navbarDropdown" data-toggle="dropdown" aria-expanded="false"> 
                                Welcome, User <b class="caret"></b>
                            </a>

                            <div class="dropdown-menu  dropdown-menu-right" aria-labelledby="navbarDropdown"><!-- ADDED CLASS -->
                                    <a class="dropdown-item" href="#">Action</a>
                                    <a class="dropdown-item" href="#">Another action</a>
                                    <div class="dropdown-divider"></div>
                                    <a class="dropdown-item" href="#">Something else here</a>
                            </div>
                                    
                    </li> 
            </ul>



    </nav>



  </body>
  </html>

CSS

.navbar {
    -webkit-box-shadow: 0 8px 6px -6px #999;
    -moz-box-shadow: 0 8px 6px -6px #999;
    box-shadow: 0 8px 6px -6px #999;

    /* the rest of your styling */
}
.navbar-nav .dropdown-menu {
  position: absolute;
}